Seat belt device

ABSTRACT

A seat belt  11  for constraining an occupant to a seat is provided with both a tension varying means  12, 16  allowing a tension given to the seat belt  11  to vary, and a tension detecting means  15, 17  for detecting a tension of the seat belt  11 . There is provided failure detecting means  18 , which detects failures of at least one of the tension varying means  12, 16  and the tension detecting means  15, 17  by comparing the tension given by the tension varying means  12, 16  to the tension detected by the tension detecting means  15, 17 , the difference in tensions thereof exceeding a predetermined value.

TECHNICAL FIELD

The present invention relates to a seat belt device that effectivelyconstrains an occupant to a seat so as to protect the occupant upon avehicle collision, and more particularly, to a seat belt device thatincludes a tension varying means allowing a tension given to a seat beltto vary and a tension detecting means for detecting a tension of theseat belt.

BACKGROUND ART

Conventionally, a seat belt device has been known in which, in order toprevent an occupant from traveling forwardly and colliding with parts ofa vehicle or the like at the time of sudden stopping of the vehicle orthe like, thereby protecting the occupant, an electromotive retractorserving as a tension varying means capable of allowing a tension givento a seat belt to vary according to a collision state is provided.

For example, Patent Document 1 (JP-A No. 2000-52925 (FIG. 1, Paragraphs0040, 0041, 0048, and 0049)) discloses a technology relating to avehicle occupant constraining and protecting device (seat belt device)having an electromotive retractor (seat belt winding device) that windsand unwinds a seat belt for protecting an occupant. In the technologydisclosed in Patent Document 1, in order to drive a seat belt windingdevice, a seat belt winding force is changed according to a tread poweron a brake pedal provided in the vehicle and a tension of the seat beltis also changed.

Further, Patent Document 2 (JP-A No. 2001-163185 (FIG. 4, FIG. 6,Paragraphs 0028, and 0041 to 0043)) discloses a technology relating to aseat belt device having a seat belt winding device that includes acollision prediction part for avoiding a collision with an obstacle,such as another vehicle in front of a vehicle. In the technologydisclosed in Patent Document 2, a distance between the vehicle and theobstacle, for example, is measured by a non-contact distance sensor,such as a laser radar, and, when the collision is likely to occur, acollision prediction signal is outputted to drive a motor, such that theseat belt winding device winds up the seat belt.

Further, a seat belt device has also been known in which a tensiondetecting means for measuring a tension given to the seat belt isprovided to confirm an attachment state of the seat belt.

For example, Patent Document 3 (JP-A No. 2002-19581 (FIG. 9 andParagraph 0009)) discloses a seat belt tension sensor having thefollowing configuration. When the tension is given to a seat belt, theseat belt wound around a seat belt winding part pulls the seat beltwinding part with respect to a supporting part. By doing so, a pivotingpart rotates or is going to rotate around a rotational center.Accordingly, a power transmission part provided at an end of thepivoting part also rotates or is going to rotate, and the one endpresses a load measuring part so that a force is exerted on the loadmeasuring part. The force exerted on the load measuring part has aconstant relationship with the tension given to the seat belt, so thatthe tension of the seat belt can be detected by detecting the force.

As regards the seat belt device including the electromotive retractorserving as the tension varying means described above, a seat belt devicethat has a failure diagnosis system of the electromotive retractoritself incorporated therein has been known.

For example, in Patent Document 4 (JP-A No. 11-170966 (FIG. 5 andParagraph 0005)), a seat belt device has been suggested in which afailure diagnosing means for diagnosing a failure of a driving meansbased on a waveform of a current flowing in the driving means, when apredetermined voltage waveform is applied to the driving means fordriving an electromotive retractor, is provided.

However, the failure diagnosing means disclosed in Patent Document 4 candetect the failure of the electromotive retractor of the tension varyingmeans, but it cannot diagnose whether or not the tension varying meansgives a predetermined tension to the seat belt.

Further, in the seat belt device having the tension detecting meansdisclosed in Patent Document 3, the failure of the tension detectingmeans cannot be diagnosed.

DISCLOSURE OF THE INVENTION

The invention has been made in consideration of the above-describedproblems, and it is an object of the invention to provide a seat beltdevice capable of detecting failures of respective means, whether or nota tension varying means gives a predetermined tension to a seat belt orwhether or not a tension detecting means correctly detects a tension ofthe seat belt.

In order to solve the above-described problems, according to a firstaspect of the invention, a seat belt device includes a seat belt forconstraining an occupant to a seat, a tension varying means allowing atension given to the seat belt to vary, a tension detecting means fordetecting a tension of the seat belt, and a failure detecting means forcomparing the tension given by the tension varying means to the tensiondetected by the tension detecting means, to detect failures of at leastone of the tension varying means and the tension detecting means.

In accordance with the first aspect of the invention, the tension givenby the tension varying means is detected by the tension detecting means,a signal corresponding to the tension to be given from the tensionvarying means to the seat belt is outputted to the failure detectingmeans, and a signal corresponding to the detected tension of the seatbelt from the tension detecting means is outputted to the failuredetecting means. The failure detecting means has a comparing means forcalculating a difference of both signals and comparing the correspondingamount to a predetermined value. When the difference is smaller than thepredetermined value, it can be judged that the tension varying means andthe tension detecting means are normal. On the other hand, when thedifference is larger than the predetermined value, it can be judged thatthe tension varying means is in failure, the tension detecting means isin failure, or both the tension varying means and the tension detectingmeans are in failure. That is, when the difference is larger than thepredetermined value, it can be judged that at least one of the tensionvarying means and the tension detecting means is in failure. Inaddition, the predetermined value is selected in consideration of a sumof a tension caused by errors of the tension varying means and thetension detecting means and a tension that is a loss from the tensionvarying means to the tension-detecting means by frictional resistance inthe respective parts of the seat belt.

According to a second aspect of the invention, in the seat belt deviceaccording to the first aspect of the invention, the seat belt may be ofthree-point support type, one end thereof being supported at anelectromotive retractor in such a manner as to be freely wound, theother end thereof being supported at a lap anchor part, and themidportion thereof being supported at a buckle stay part. The tensionvarying means may be provided at the electromotive retractor. Thetension detecting means may be provided at the lap anchor part.

In accordance with the second aspect of the invention, when the tensionvarying means is provided at one end of the three-point support typeseat belt, the tension detecting means is provided at the other end ofthe three-point support type seat belt. Therefore, the failures of atleast one of the tension varying means and the tension detecting meanscan be detected regardless of whether or not the seat belt is attachedto in the buckle stay part, that is, regardless of attachment/detachmentof the seat belt.

According to a third aspect of the invention, in the seat belt deviceaccording to the first aspect of the invention, the seat belt may be ofthree-point support type, one end thereof being supported at anelectromotive retractor in such a manner as to be freely wound, theother end thereof being supported at a lap anchor part, and themidportion thereof being supported at a buckle stay part. The tensionvarying means and the tension detecting means may be arranged such thatthe tension varying means is provided at the retractor and the tensiondetecting means is provided at the buckle stay part, such that thetension varying means is provided at the buckle stay part and thetension detecting means is provided at the lap anchor part, or such thatthe tension varying means and the tension detecting means are providedat the buckle stay part. That is, at least one of the tension varyingmeans and the tension detecting means is provided at the buckle staypart. The buckle stay part may be provided with an attachment/detachmentdetecting means for detecting an attachment of the seat belt. Further,the failure detecting means may be allowed to be actuated when theattachment/detachment detecting means is detecting the attachment of theseat belt.

In accordance with the third aspect of the invention, at least one ofthe tension varying means and the tension detecting means is provided atthe buckle stay part. Therefore, when the attachment of the seat beltdevice is not detected by the attachment/detachment detecting means, thefailure detection cannot be performed. That is, with the attachment ofthe seat belt, failures of at least one of the tension varying means andthe tension detecting means can be detected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing an example of a seat beltdevice according to a first embodiment of the invention;

FIG. 2 is a diagram schematically showing a configuration of anelectromotive retractor;

FIG. 3 is a diagram showing a structure of a lap anchor part into whicha tension detecting means is incorporated;

FIG. 4 is a cross-sectional view taken along the line IV-IV of FIG. 3;

FIG. 5 is a cross-sectional view taken along the line V-V of FIG. 3;

FIG. 6 is a cross sectional view taken along the line VI-VI of FIG. 3;

FIG. 7 is a diagram schematically showing a configuration of a failuredetecting means according to the first embodiment of the invention;

FIG. 8 is a flowchart showing a sequence of a control program of thefailure detecting means according to the first embodiment of theinvention;

FIG. 9 is a diagram schematically showing a configuration of anelectromotive belt winding device according to a second embodiment ofthe invention;

FIG. 10 is a diagram schematically showing a configuration of a failuredetecting means according to the second embodiment of the invention;

FIG. 11 is a flowchart showing a sequence of a control program of thefailure detecting means according to the second embodiment of theinvention;

FIG. 12 is a diagram schematically showing a configuration of anelectromotive belt winding device according to a third embodiment of theinvention;

FIG. 13 is a perspective view of a tension varying means;

FIG. 14 is a side view of the tension varying means;

FIG. 15 is a cross-sectional view taken along the line 5-5 of FIG. 14;

FIG. 16 is an exploded view of essential parts of the tension varyingmeans; and

FIG. 17 is a diagram schematically showing a configuration of anelectromotive belt winding device according to a fourth embodiment ofthe invention.

BEST MODE FOR CARRYING OUT THE INVENTION FIRST EMBODIMENT

A seat belt device according to a first embodiment of the invention willbe described with reference to FIG. 1. FIG. 1 is a diagram schematicallyshowing an example of a seat belt device according to the firstembodiment of the invention.

As shown in FIG. 1, the seat belt device 10 is of a three-point supporttype in which a seat belt 11 for constraining an occupant 1 to a seat 2,an electromotive retractor 12 installed on a side surface of a vehiclebody 3 for winding one end of the seat belt 11 to allow a tension tovary, a shoulder through 13 installed on an upper side of the sidesurface of the vehicle body 3 for turning down the seat belt 11 in thevicinity of a shoulder of the occupant 1, a buckle stay part 14 fordetachably engaging the midportion of the seat belt 11 at a lower sideof the vehicle body 3 in the vicinity of a waist of the occupant, and alap anchor part 15 for fixing the other end of the seat belt 11 on alower side of the side surface of vehicle body 3.

The buckle stay part 14 includes a through tongue 14 a for turning downthe seat belt 11 and a buckle 14 b installed on the lower side of thevehicle body to be detachably engaged with the through tongue 14 a.

The seat belt device 10 further includes a tension detecting means 17provided at the lap anchor part 15 and a failure detecting means 18connected to a control unit 16 for controlling a motor of theelectromotive retractor 12 and to the tension detecting means 17.

The electromotive retractor (electromotive belt winding device) 12includes the control unit 16 and a tension varying means. Theelectromotive retractor 12 will be described with reference to FIGS. 2and 3. FIG. 2 is a diagram schematically showing the configuration ofthe electromotive retractor 12. FIG. 3 is a diagram schematicallyshowing an example of a potentiometer.

As shown in FIG. 2, the electromotive retractor 12 includes a frame 101,a seat belt locking mechanism 102, a reel 103, a pre-tensioner 104,pulleys 105 and 106, a power transmission belt 107, a direct current(DC) motor (motor) 108, a potentiometer 109, a seat belt winding spring110, and an electromagnetic actuator 111. In addition, as shown in FIG.2, the electromotive retractor 12 is connected to the control unit 16.

The reel 103 and a reel shaft 103 a are provided at the frame 101. Thereel 103 is to wind the seat belt 11. The reel shaft 103 a serves as acenter axis of a reel rotation and has a left end coupled to the reel103 and a right end coupled to the seat belt locking mechanism 102,which is described below. In addition, the reel shaft 103 a is a twistedaxis and serves as an energy absorption means. That is, in a state inwhich the right end of the reel shaft 103 a is locked by the seat beltlocking mechanism 102, when the seat belt 11 is unwound by an intensiveforce and the reel 103 rotates, the reel shaft 103 a itself is twistedaround the axis and plastically deformed. Therefore, the seat belt 11 isunwound so that the seat belt 11 absorbs impact energy acting on a bodyof the occupant.

The seat belt locking mechanism 102 is to lock unwinding of the seatbelt 11 and includes a VSI operation to lock unwinding of the seat belt11 when a predetermined deceleration is exerted on the vehicle, and aWSI operation to lock unwinding of the seat belt 11 when the seat belt11 is unwound with a predetermined acceleration. In addition, the seatbelt locking mechanism 102 may include an electromagnetic actuator 111,which is described below. Further, even when unwinding of the seat belt11 is locked, the seat belt locking mechanism 102 is arranged such thatthe seat belt 11 can be wound by the direct current motor 108, which isdescribed below.

The pre-tensioner 104 operates by an output of the collision detectingpart (not shown) via the control unit 16 to rotate the reel shaft 103 ain a retraction direction of the seat belt 11 and to constrain theoccupant to the seat by forcibly winding the seat belt 11. Thepre-tensioner 104 includes a pre-tensioner squib, such as a powder-typepre-tensioner or the like. For example, the pre-tensioner 104 includes atransmission mechanism for changing a piston movement caused by a gaspressure in a cylinder, in which a gas generated from a gas generator issealed, into a rotational movement of the reel shaft 103 a via a clutchmechanism.

The pulley 105 is fixed to the reel shaft 103 a, and the pulley 106 isfixed to a shaft of the direct current motor 108, which is describedbelow. The power transmission belt 107 is to connect the pulley 105 andthe pulley 106. A predetermined number of external teeth are formedrespectively at outer circumferences of the pulley 105 and the pulley106, a predetermined number of internal teeth are also formed at aninner circumference of the power transmission belt 107, and a pluralityof teeth for the pulley 105, the pulley 106, and the power transmissionbelt 107 are geared one another in exact quantities.

The direct current motor 108 is fixed to the frame 101 at two points ormore and is operated by an output of the control unit 16. The rotationof the direct current motor 108 is transmitted to the reel shaft 103 athrough the pulley 106, the power transmission belt 107, and the pulley105. Therefore, the direct current motor 108 rotates forwardly to windthe seat belt 11, and the direct current motor 108 rotates inversely tounwind the seat belt 11. In addition, the number of rotation of thedirect current motor 108 is proportional to a current supplied to thedirect current motor 108, and when the direct current motor 108 rotatesforwardly, the current supplied to the direct current motor 108 isproportional to a tension of the seat belt 11. Accordingly, theelectromotive retractor 12 and the control unit 16 constitute thetension varying means that can allow a tension given to the seat belt 11to vary.

In addition, the value of a current flowing in the direct current motor108 is detected as the value of a voltage corresponding to the currentby a current detector provided at the control unit 16 serving as a motordriving circuit and is outputted to the failure detecting means 18,which is described below. The current of the direct current motor 108has relation to a rotational torque of the direct current motor 108, sothat the rotational torque, that is, the tension given to the seat belt11, can be estimated by a load current value.

The potentiometer 109 is provided at the leftmost end of the reel shaft103 a, and includes a resistor (not shown) with both ends applied to avoltage and a slide member (not shown) geared with a rotation of thereel shaft 103 a. Therefore, the voltage corresponding to a rotationalamount from a reference position of the reel shaft 103 a is outputted tothe control unit 16, which is described below, and thus, for example, anunwinding amount of the seat belt 11 and a loosening amount of the seatbelt 11 are estimated.

The seat belt winding spring 110 is to house the seat belt 11 in theelectromotive retractor (electromotive belt winding device) 12 by aspring force, when the seat belt 11 is not attached. The electromagneticactuator 111 is, for example, a solenoid, and forcibly operates the seatbelt locking mechanism 102 in response to an instruction signal of theseat belt locking mechanism 102. An operation of the electromagneticactuator 111 is controlled by an output of the control unit 16, which isdescribed below.

The lap anchor part 15 has the tension detecting means 17 incorporatedtherein. When the buckle stay part 14 is in an engagement state asshown, when the tension is exerted on the seat belt 11 from theelectromotive retractor 12, a tension corresponding to the tensionacting on the seat belt 11 is exerted on the lap anchor part 15. In themeanwhile, even when the buckle stay part 14 is not in an engagementstate as shown, when the tension is exerted on the seat belt 11 from theelectromotive retractor 12, a tension corresponding to the tensionacting on the seat belt 11 is exerted on the lap anchor part 15. Thistension can be detected by the tension detecting means 17 provided atthe lap anchor part 15.

The configuration of the lap anchor part 15 having the tension detectingmeans 17 incorporated therein will be described with reference to FIG.3, FIG. 4, which is a cross-sectional view taken along the line IV-IV ofFIG. 3, and FIG. 5, which is a cross-sectional view taken along the lineV-V of FIG. 3. In addition, FIG. 6 is a cross-sectional view taken alongthe line VI-VI of FIG. 3.

The lap anchor part 15 is provided in a mounting structure of an anchorplate 211 and a base plate 220. The mounting structure includes ananchor bolt 215, a plastic washer 216 and a plastic L bush 217 as anabrasion and noise-prevention member, a wave washer 218 as a biasmember, and an L bush 219 as a flange member. These members fix the baseplate 220 to the vehicle body 3 and movably installed the anchor plate211 on the base plate 220. The base plate 220 is a member having aU-shaped section and includes a backboard 220 a and a pair of sideboards220 b and 220 b. A fixing hole 220 c, through which the anchor bolt 215passes, is provided at the backboard 220 a. The anchor plate 211 isslidably fit between the sideboards 220 b and 220 b of the base plate220.

The anchor bolt 215 includes a flange part 215 b that is disposedneighboring to a head part 215 a and that has a diameter larger than thehead part 215 a, a step part 215 c that is disposed neighboring to theflange part 215 b and that has a diameter smaller than the flange part215 b, and a shaft part 215 d that is disposed neighboring to the steppart 215 c and that has a diameter smaller than the step part 215 c. Thestep part 215 c passes through a long hole 211 b of the anchor plate 211and the shaft part 215 d passes through a fixing hole 220 a of the baseplate 220 while being threaded into the vehicle body. The plastic washer216 is fit outwardly into the step part 215 c of the anchor bolt 215. Inthe plastic L bush 217 having a cylinder part and a flange part, thecylinder part is fit outwardly into the step part 215 c and the cylinderpart is slidably fit inwardly into the long hole 211 b of the anchorplate 211. The flange part of the plastic L bush 217 is interposedbetween the anchor plate 211 and the backboard 220 b of the base plate220. With the plastic washer 216 and the plastic L bush 217, when theanchor plate 211 slides toward the base plate 220, sliding can beensured between the anchor bolt 215 and the anchor plate 211 and betweenthe anchor plate 211 and the base plate 220, while preventing noise andscraping caused by a metal contact.

The L bush 219 includes a cylinder part and a flange part, and the wavewasher 218 is fit outwardly into the cylinder part. An outer diameter ofthe wave washer 218 is larger than that of the fixing hole 220 c of thebase plate 220. When the anchor bolt 215 is threaded into the vehiclebody, a front end of the cylinder part of the L bush 219 comes incontact with the step part 215 c of the anchor bolt 215. The wave washer218 and the L bush 219 are interposed between the backboard 220 a of thebase plate 220 and the vehicle body. The base plate 220 and the anchorplate 211 are rotatably installed on the vehicle body with the anchorbolt 215 as a center. The wave washer 218 is constantly biased in adirection to separate the base plate 220 from the vehicle body, andthus, at the time of the rotation of the base plate 220, the noise canbe prevented.

As shown in FIG. 5, one end of the backboard 220 a has a wire guide part220 d disposed approximately at a center between the sideboards 220 band 220 b. In the wire guide part 220 d of the base plate 220, a throughhole for a flexible wire 225 is provided at an end wall that rises fromthe backboard 220 a and protrudes to an upper side than the anchor plate211. The flexible wire 225 has a metal core member 225 a havingflexibility and an outer tube 225 b as a covering material to cover thecore member 225. The outer tube 225 b is made of a bulge part 225 c as astopping part that allows a portion neighboring to an outer side of thewire guide part 220 d to be bent. The end of the core member 225 a stopson an upper surface of the anchor plate 211 by a locking member 226. Aguide convex part 211 c coming in contact with the backboard 220 a isprovided on a lower surface of the anchor plate 211 (a surface facingthe backboard 220 a of the base plate 220) on an opposite side to a sidewhere the seat belt 11 is engaged (right side in the drawing).

With the tension larger than a predetermined value given to the seatbelt 11, the anchor plate 211 may be pulled by the seat belt 11 in anarrow P1 direction (left direction in the drawing), which is a surfacedirection of the anchor plate 211, or in an arrow P2 direction, which isa tilted direction toward a direction to be separated from the vehiclebody with respect to the surface direction of the anchor plate 211. Inthis case, only the anchor plate 211 is guided between the sideboards220 b of the base plate 220 to move in the arrow P1 direction.

That is, while the anchor bolt 215 and the base plate 220 do not move,the plastic L bush 217 fit outwardly into the anchor bolt 215 relativelyslides into the long hole 211 b of the anchor plate 211. When the anchorplate 211 is pulled to a center and an upper side of a seat back, theanchor plate 211 is pulled in the arrow P2 direction shown in FIG. 5. Inthis case, the anchor plate 211 restricts the movement toward the upperside in FIG. 5 using the plastic washer 216 coming in contact with theflange part 215 b of the anchor bolt 215. Further, since the guideconvex part 211 c comes in contact with the backboard 220 a of the baseplate 220, the anchor plate 211 smoothly moves in the arrow P1direction, not being inclined in the arrow P2 direction.

The structure of the tension detecting means 17 incorporated into thelap anchor part 16 will be described with reference to FIG. 6. Thetension detecting means 17 is incorporated into a unit 230 that has anupper case 231 and a lower case 232 to be integrated. The lower case 232can be formed through a press process or the like and includes abackboard 232 a and a sideboard 232 b. In the lower case 232, a coilspring 233 serving as a bias member and a cap member 234 serving as amoving member are housed. The cap member 234 includes a cylinder cupmember 234 a having a bottom and a lid part 234 b serving as a contactpart. The lid part 234 b can be formed through molding or the like.

The coil spring 233 is interposed between a bottom board of the cup part234 a of the cap member 234 and the sideboard 232 b of the lower case232. A sensing portion, the shaft 238 a, of a linear potentiometer 238comes in contact with a surface of the lid part 234 b, which is insertedinto an opening of the cup part 234 a, on an opposite side to the sideof the coil spring 233. An axial direction of the shaft 238 aapproximately matches an axial direction of the coil spring 233. The lidpart 234 b of the cap member 234 is provided with amisalignment-prevention convex part 234 c to surround a front end of theshaft 238 a. The linear potentiometer 238 is fixed to the lower case 232through a screw stopper or the like. The upper case 231 includes a topboard 231 a to cover the lower case 232 and the linear potentiometer238, and a sideboard 231 b. The upper case 231 can be formed through apress deep drawing process or plastic die cast.

In order to allow the core member 225 a and the outer tube 225 b of theflexible wire member 225 to pass through the sideboard 231 b of theupper case 231 and the sideboard 232 b of the lower case 232, U-shapednotches (not shown) are provided in the sideboards 231 b and 232 b toface each other, respectively. In the outer tube 225 b of the flexiblewire member 225, a portion neighboring to the outer side of thesideboard 231 b of the upper case 231 is bent to form the bulge part 225c. The core member 225 a passes through the coil spring 233 in an axialdirection and passes through the bottom board of the cup part 234 a tobe terminated within the cup part 234 a. A terminal locking part 225 dhaving a diameter larger than the core member 225 a is provided at theend part of the core member 225 a. At an initial state, as shown in thedrawing, a gap S is provided between the terminal locking part 225 ddisposed in the cup part 234 a and the bottom board of the cup part 234a. In addition, the coil spring 233 is not expanded and contracted.

The linear potentiometer 238 is connected to the failure detecting means18, as shown in FIG. 1.

Returning to FIG. 3, the operation of the tension detecting means 17 isdescribed. The flexible wire member 225 arranged between the anchorplate 211 and the unit 230 has a surplus length, so that the anchorplate 211 and the base plate 220 can turn the anchor bolt 215 accordingto the motion of a webbing W by a predetermined amount with the anchorbolt 215 as a center, as indicated by the arrow R. When the anchor plate211 and the base plate 220 rotate, the core member 225 a of the flexiblewire member 225 may be pulled out by the anchor plate 211. However, asshown in FIG. 6, since the gap S is provided between the terminallocking part 225 d of the core member 225 a and the cup part 234 a,there is no case in which the linear potentiometer 238 erroneouslyoperates due to disturbance, as described above.

Therefore, when the predetermined tension acts on the seat belt 11, theanchor plate 211 is pulled out by the seat belt 11, and thus the anchorplate 211 relatively moves with respect to the vehicle body. Thus, inthe unit 230 shown in FIG. 6, the terminal locking part 225 d of theflexible wire member 225 comes in contact with the bottom board of thecup part 234 a, while the cap member 234 is pulled by the flexible wiremember 225 and moves due to a biasing force. Accordingly, the shaft 238a of the linear potentiometer 238 stretches and a change in voltage(current) corresponding to a relative movement between the anchor plate211 and the vehicle body is outputted to the failure detecting means 18,which is described below. The linear potentiometer 238 measures apositional change of the cap member 234, such as a change in terminalposition of the coil spring 233, rather than a positional change of theterminal locking part 225 d of the flexible wire member 225, so that thetension acting on the seat belt 11 can be correctly detectedelectrically and linearly.

Next, the failure detecting means 18 according to the first embodimentof the invention will be described with reference to FIGS. 7 and 8. FIG.7 is a diagram schematically showing the configuration of the failuredetecting means 18 according to the first embodiment of the invention.FIG. 8 is a flowchart showing the sequence of a control program of thefailure detecting means.

As shown in FIG. 7, the failure detection means 18 includes a CPU(central processing unit) 301, a ROM (read only memory) 302, a RAM(random access memory) 303, an input interface 304, and an outputinterface 305.

The CPU 301 loads a control program and data stored in the ROM 302 intoa work area of the RAM 303 and controls an operation of a warning lamp19. The input interface 304 is connected to the tension varying means 12and the tension detecting means 17. The output interface 305 isconnected to the warning lamp 19 and an airbag control ECU 20.

Next, a control program concerning the failure detecting means 18according to the first embodiment of the invention will be describedwith reference to FIG. 8. In addition, the CPU 301 periodically executesthe control program, and thus the tension Mt detected from the tensiondetecting means 17 is read out (Step S1). Further, the tension St set bythe tension varying means 12 is read out (Step S2).

Next, a difference Δ between the detected tension Mt and the set tensionSt is calculated and the difference is compared to a predetermined valueTr (Step S3). When the difference Δ is smaller than the predeterminedvalue Tr (Step S3: YES), it is judged that it is normal, a normal resultis outputted, and the control program ends (Step S4). When thedifference Δ is larger than the predetermined value Tr (step S3: NO), itis judged that it is abnormal, an abnormal result is outputted, and thecontrol program ends (Step S5). When the abnormal result is outputted,the warning lamp 19 turns on through the output interface 305, as shownin FIG. 6. In addition to or instead of turning on the lamp 19, anoperation of the airbag is controlled through the airbag control ECU 20according to the failure state of the seat belt device.

The predetermined value is selected as a value larger than a sum of thetension generated by an error of each of the tension varying means 12and the tension detecting means 17, and the tension caused by frictionalresistance of each part of the seat belt 11, such as the shoulderthrough 13, the buckle stay part, or the like, which is a loss from thetension varying means 12 to the tension detecting means 17.

In the seat belt device 10 shown in FIG. 1, the tension varying means 12is provided at one end of the seat belt 11 and the tension detectingmeans 17 is provided at the other end of the seat belt 11. Therefore, asshown in FIG. 1, when the seat belt 11 is attached to the occupant andthe tension varying means 12 gives an appropriate tension to the seatbelt 11, the given tension is outputted as the set tension, and theoutput from the tension detecting means 17 is used as the detectedtension, such that both can be compared to each other. In addition, evenwhen the seat belt 11 is not attached, the tension varying means 12gives an appropriate tension to the seat belt 11, the given tension isoutputted as the set tension and the output from the tension detectingmeans 17 is used as the detected tension, such that both can be comparedto each other. As such, failures of one or both of the tension varyingmeans 12 and the tension detecting means 17 can be detected, regardlessof whether or not the seat belt is attached. Therefore, the detachmentstate of the seat belt is confirmed by the seat beltattachment/detachment detecting means, and, by operating the tensionvarying means 12, the tension can be detected by the tension detectingmeans 17. In this case, the loss of the tension due to friction or thelike of the buckle stay part 14 is lowered, and the predetermined valueTr is reduced with respect to the difference Δ between the detectedtension Mt and the set tension St, such that precision of the failuredetection can be enhanced.

SECOND EMBODIMENT

Next, a seat belt device 30 according to a second embodiment of theinvention will be described with reference to FIGS. 9 to 11. FIG. 9 is adiagram schematically showing the configuration of an electromotive beltwinding device according to the second embodiment of the invention.

The seat belt device 30 is of a three-point support type and includes aseat belt 11 for constraining an occupant 1 to a seat 2, anelectromotive retractor 12 installed on a side surface of a vehicle body3 for allowing a tension to vary by winding one end of the seat belt 11,a shoulder through 13 installed on an upper side of the side surface ofthe vehicle body 3 for turning down the seat belt 11 in the vicinity ofa shoulder of the occupant 1, a buckle stay part 31 for removablyengaging a midportion of the seat belt 11 on the lower side of thevehicle body 3 in the vicinity of a waist of the occupant, and a lapanchor part 32 for fixing the other end of the seat belt 11 on a lowerside of the side surface of vehicle body 3.

The lap anchor part 32 is of a typical type and is fixed to the vehiclebody. The buckle stay part 31 includes a through tongue 33 for turningdown the seat belt 11, and a buckle 34 installed on the lower side ofthe vehicle body to be detachably engaged with the through tongue 33.

The seat belt device 30 further includes a tension detecting means 35provided at the buckle 34 of the buckle stay part 31, a failuredetecting means 36 connected to a control unit 16 for controlling amotor of the electromotive retractor 12 and to the tension detectingmeans 35, and an attachment/detachment detecting means (buckle switch)37 for detecting an attachment/detachment of the through tongue 33.

The electromotive retractor (electromotive belt winding device) 12includes the control unit 16 and the tension varying means. Here, theelectromotive retractor 12 has the same parts as those in the firstembodiment.

As described in the first embodiment, the tension detecting means 35includes a mounting structure of the anchor plate 211 and the base plate220, and a unit 230 in which the upper case 231 and the lower case 232are incorporated in the same manner. That is, the tension about twice aslarge as the tension given to the seat belt 11 from the electromotiveretractor 12 is detected by the tension detecting means 35. The tensioncan be detected in a state in which the seat belt 11 is attached to thebuckle stay part 31. For this reason, the attachment/detachmentdetecting means 37 of the seat belt 11 is provided at the buckle 34.

Next, the failure detecting means 36 according to the second embodimentof the invention will be described with reference to FIGS. 10 and 11.FIG. 10 is a diagram schematically showing the configuration of afailure detecting means 36 according to the second embodiment of theinvention. FIG. 11 is a flowchart showing a sequence of a controlprogram of the failure detecting means 36 according to the secondembodiment of the invention.

As shown in FIG. 10, the failure detection means 36 includes a CPU(central processing unit) 351, a ROM (read only memory) 352, a RAM(random access memory) 353, an input interface 354, and an outputinterface 355.

The CPU 351 loads a control program and data stored in the ROM 352 intoa work area of the RAM 353, and controls an operation of a warning lamp19. The input interface 354 is connected to the tension varying means12, the tension detecting means 35, and the attachment/detachmentdetecting means 37. The output interface 355 is connected to the warninglamp 19 and an airbag control ECU 20.

Next, a control program concerning the failure detecting means 36according to the second embodiment of the invention will be describedwith reference to FIG. 11. In addition, the CPU 351 periodicallyexecutes the control program and judges whether or not the seat belt 11is attached (Step S1). Here, whether or not the seat belt 11 is attachedis judged with on/off of an attachment flag inputted from the buckleswitch 37 via the input interface 354.

Next, the tension Mt detected from the tension detecting means 35 isread out (Step S12). Further, the tension St set by the tension varyingmeans 12 is read out (Step S13).

Next, a difference Δ between the detected tension Mt and the set tensionSt is calculated, and the difference is compared to a predeterminedvalue Tr (Step S14). When the difference Δ is smaller than thepredetermined value Tr (Step S14: YES), it is judged that it is normal,a normal result is outputted, and the control program ends (Step S15).When the difference Δ is larger than the predetermined value Tr (StepS14: NO), it is judged that it is abnormal, an abnormal result isoutputted, and the control program ends (Step S16). When the abnormalresult is outputted, the warning lamp 19 turns on through the outputinterface 355, as shown in FIG. 9. In addition to or instead of turningon the lamp 19, an operation of the airbag is controlled through theairbag control ECU 20 according to the failure state of the seat beltdevice.

The predetermined value is selected as a value larger than a sum of thetension generated by an error of each of the tension varying means 12and the tension detecting means 35, and the tension caused by frictionalresistance of each part of the seat belt 11, such as the shoulderthrough 13, the buckle stay part 31, or the like, which is a lossgenerated when the tension is transmitted from the tension varying means12 via the buckle stay part 31 to the tension detecting means 31.

In the seat belt device 30 of FIG. 9, the tension varying means 12 isprovided at one end of the seat belt 11, and the tension detecting means17 is provided at the buckle stay part 31 of the midportion of the seatbelt 11. Therefore, as shown in FIG. 9, when a buckle sensor 37 detectsthat the seat belt 11 is attached to the occupant and the tensionvarying means 12 gives an appropriate tension to the seat belt 11, thegiven tension is outputted as the set tension and the output from thetension detecting means 35 is used as the detected tension, such thatboth can be compared to each other.

THIRD EMBODIMENT

Next, a seat belt device 50 according to a third embodiment of theinvention will be described with reference to FIGS. 12 to 16. FIG. 12 isa diagram schematically showing the configuration of an electromotivebelt winding device according to the third embodiment of the invention.

The seat belt device 50 is of a three-point support type and includes aseat belt 11 for constraining an occupant 1 to a seat 2, a retractor 51installed on a side surface of a vehicle body 3 for winding one end ofthe seat belt 11, a shoulder through 13 installed on an upper side ofthe side surface of the vehicle body 3 for turning down the seat belt 11in the vicinity of a shoulder of the occupant 1, a buckle stay part 52for detachably engaging a midportion of the seat belt 11 on the lowerside of the vehicle body 3 in the vicinity of a waist of the occupant,and a lap anchor part 53 for fixing the other end of the seat belt 11 ona lower side of the side surface of vehicle body 3.

The buckle stay part 52 includes a through tongue 54 for turning downthe seat belt 11, and a buckle 55 installed on the lower side of thevehicle body to be detachably engaged with the through tongue 54.

The seat belt device 50 further includes a tension detecting means 56provided at the lap anchor part 53, a tension varying means 57 providedat the buckle 55 of the buckle stay part 52, a failure detecting means58 connected to the tension varying means 57 and the tension detectingmeans 56, and an attachment/detachment detecting means (buckle switch)37 provided at the buckle 55 for detecting the attachment/detachment ofthe through tongue 54.

The retractor (belt winding device) 51 has the same configuration asthat described in the first embodiment, except that the belt is woundwith the predetermined tension, not being varied. That is, a forciblelocking mechanism and a pre-tensioner are included. An electromotivetype is preferably, but the retractor may not be the electromotive type.

As described in the first embodiment, the tension detecting means 56includes a mounting structure of the anchor plate 211 and the base plate220, and a unit 230 in which the upper case 231 and the lower case 232are incorporated in the same manner. That is, the tension given by thetension varying means 57 described below is detected by the tensiondetecting means 56. The tension cannot be detected when the seat belt 11is not in an attachment state with respect to the buckle stay part 52.For this reason, the attachment/detachment detecting means 37 of theseat belt 11 is provided at the buckle 55, as in the same manner withthe second embodiment.

Next, the configuration of the tension varying means 57 (locking means Lhaving a reversible configuration) arranged between the buckle 55 andthe vehicle body will be described with reference to FIGS. 13 to 16.FIG. 13 is a perspective view of the tension varying means. FIG. 14 is aside view of the tension varying means. FIG. 15 is a cross-sectionalview taken along the line 5-5 of FIG. 14. FIG. 16 is an exploded view ofprimary components.

The tension varying means 57 shown in FIGS. 13 and 14 includes a buckle55, into which the through tongue 54 of the seat belt 11 is inserted tobe engaged and which is movable in a retraction direction and aprotraction direction of the seat belt 11, a rail 415 arranged along themoving direction of the buckle 55, a locking means L that has a knuckle407 being movably guided along the rail 415, together with the buckle 55and having a rotational center and that blocks the movement of thebuckle 55 in the protraction direction by causing the knuckle 407 torotate and to be bit into the rail 415, a wire 421 that can be engagedin both the retraction and protraction directions with respect to thebuckle 55 and move the locking means L in the retraction or protractiondirection by allowing to be driven in any one direction, anelectromotive winch 460 with respect to the wire 421, and a movementauxiliary member 450 that reduces movement resistance in the retractiondirection of the locking means L between the locking means L and therail 415. The electromotive winch 460 includes a motor 461 and a reel462. The motor 461 is driven and controlled by the control unit 463.

The buckle 55 has a button 403 for releasing the engagement of thethrough tongue 54 and is fixed in a state in which an engagement opening401 is directing toward a front end 405 a of a support bar 405. A baseend 405 b of the support bar 405 is fixed to the knuckle 407 serving asa movable part that moves integrally with the buckle 55. In the knuckle407, one end 407 a thereof is coupled to the support bar 405 at aboutright angles, and the other end part 407 b serving as a locking part isinserted between two sideboards 409 a and 409 a of a slider 409 having aU-shaped section. On both sideboards 409 a and 409 a of the slider 409having the U-shaped section, a center pin (rotational center) 411 passesthrough as a rotational shaft, and the midportion 407 c of the knuckle407 is rotatably connected to the center pin 411. Therefore, when aforce is exerted on the buckle 55 in the retraction direction, asindicated by an arrow A of FIG. 14 (in the front end direction of thesupport bar 405), a rotational momentum is exerted on the knuckle 407 ina counterclockwise direction in the drawing, with a center of the centerpin 411 as a rotational shaft.

The slider 409 is inserted outside the rail 415 serving as a non-movableguide member having a bracket 413 installed on the end and is supportedslidably along the rail 415. The rail 415 has a mounting bracket 413fixed to the vehicle body and the seat, and is arranged along alongitudinal direction of the vehicle body. The buckle 55 is slidablysupported through the knuckle 407, the center pin 411 and the slider409, in the front direction of the vehicle body (the arrow A direction)of the seat belt protraction direction, which is the first direction,and in the rear direction of the vehicle body (the arrow B direction) ofthe seat belt retraction direction, which is the second direction, alongthe rail 415.

The rail 415 has sideboards 415 a and 415 a at both ends in thewidthwise direction, and the other end part 407 b of the knuckle 407 isdisposed between two sideboards 415 a and 415 a. Therefore, as shown inFIG. 14, the force in the front direction of vehicle direction (Adirection) is exerted on the buckle 55, the rotational momentum isgenerated. When the knuckle 407 rotates in the counterclockwisedirection, a point P₁ at an outer circumference of the other end part 7b of the knuckle 407 is bit into the bottom wall 415 b of the rail 415,and thus the movement of the knuckle 407, that is, the movement of thebuckle 55, can be blocked.

In this case, the bite point P₁ is located at the front side of thevehicle body in a vertical line 417 down to the bottom wall 415 b of therail 415 from a center of the center pin 41, as shown in FIG. 14.Accordingly, as a retraction force in the front direction of the vehiclebody is exerted on the buckle 55, the bite operation becomes tighter anda locking force becomes larger. At this time, the knuckle 407, thecenter pin 411, and the rail 415 constitute the locking means L forpreventing the movement of the buckle 55.

In addition, in the rail 415, one end of the wire (driving wire) 421 iswired to move the knuckle 407 in the longitudinal direction of thevehicle body of the A direction or the B direction. The wire 421 isarranged along the rail 415, as shown in FIG. 14, and located at about amiddle point between the center pin 411 and the cut-into point P₁. Thewire 421 moves in the A direction and in the B direction, with anactuator of the motor connected to the base end, so that the front end421 a thereof is inserted into a through hole 427 passing through andbeing formed in the front end 407 b of the knuckle 407, and two hooks423 and 425 engaged with the knuckle 407 are fixed.

The front-end hook 423 is a retracting hook for the retraction movementof the buckle 55 in the rear direction of the vehicle body (in the Bdirection), and the other hook 425 is a protracting hook for aprotraction movement of the buckle 55 in the front direction of thevehicle body (in the A direction), such that the buckle 55 can bereversibly driven with the hooks 423 and 425.

The retracting hook 423 has a clinch part 423 a protruding toward thecenter pin 411, and a side surface in the B direction of the clinch part423 a faces an engagement wall 429 formed at the end of the through hole427 of the knuckle 407. Therefore, when the wire 421 is driven in the Bdirection and the retracting hook 423 is moved, an engagement operationpoint P₁ of the above-described side surface of the clinch part 423 acomes in contact with the engagement wall 429 of the knuckle 7, so thatthe force in the retraction direction of the seat belt (force in the Bdirection) is transmitted from the retracting hook 423 to the other end407 b of the knuckle 407. In this case, the engagement operation pointP₂ is disposed between the center line (core wire) 431 of the wire 421and the center pin (rotational center) 411, and also is disposed on thecenter pin 411 (rotational center) while being spaced by a distance froma center line 431 of the wire 421.

In addition, when the protracting hook 425 has the clinch part 425 aprotruding opposite to the center pin 411, a protrusion 425 b disposedat the front end of the side surface facing the A direction of theclinch part 425 a faces an engagement wall 433 toward the B direction ofthe knuckle 407. Therefore, when the wire 421 is driven in the Adirection and the protracting hook 425 is moved, an engagement operationpoint P₃ of the front end of the protrusion 425 b comes in contact withthe engagement wall 433 of the knuckle 407, and thus the force in theseat belt retraction direction (force in the A direction) is transmittedto the other end 407 b of the knuckle 407 from the protracting hook 425.In this case, the engagement operation point P₃ is disposed on anopposite side to the center pin (rotational center) 411 with the wire421 interposed therebetween while being spaced by a distance from thecenter line 431 of the wire 421.

In addition, as shown in FIG. 14, stopper parts 435 are provided at thesideboards 415 a and 415 a of the rail 415 to define a movement limit ofthe slider 409 and the knuckle 407. The stopper parts 435 are disposedto come in contact with the center pin 411.

Further, the movement auxiliary member 450 is provided, as shown inFIGS. 15 and 16. That is, the movement auxiliary member 450 includes asupport member 451 and a contact body 453 and is supported by the slider409. Specifically, the bottom wall 409 b of the slider 409 is formed tomaintain a predetermined interval with respect to the bottom wall 415 bof the rail 415. The support member 451 and the contact body 453 aredisposed between the bottom walls 409 b and 415 b.

The support member 451 is provided with a support shaft, and deformableparts 451 b are provided at both ends 451 a thereof. The deformableparts 451 b are arranged to have a diameter smaller than those of otherparts of the support member 451. Thus, both ends 451 a are supported tosupport holes 455 provided at two locations of the sidewalls 409 a ofthe slider 409. As for the support, snap rings (not shown) are fittedinto both ends 451 a and thus loosening is prevented.

The above-described contact body 453 may be made of one of high-densitypolyethylene, polytetrafluoro ethylene, polyamide, and polyacetal.Alternatively, the contact body 453 may be made of a hollow cylindricalbody, which is formed by coating one of polyethylene, polytetrafluoroethylene, polyamide, and polyacetal on a surface of a metal moldproduct. The inner circumference 453 a of the contact body 453 is formedto have an outer diameter larger than those of other parts of thesupport member 451, is loosely fitted into the support member 451 tofreely rotate.

Next, an operation of the tension varying means 57 will be described. Inthe case of emergency, when an emergency condition occurs due to asudden deceleration to a certain level or more, the retractor (notshown) is locked, the motor (not shown) rotates forwardly according tothe detection of the emergency condition, and the wire 421 isinstantaneously retracted in the seat belt retraction direction B. Withthe retraction operation, the clinch part 423 a of the hook 423 isengaged with the engagement wall 429, and the knuckle 407 is moved inthe B direction. With this movement, the slider 409 is guided into therail 415, and the locking means L is moved in the B direction as awhole. As such, the buckle 55 is retracted in the same direction toretract the seat belt 11 in the same direction via the through tongue 54by a predetermined amount. Therefore, looseness of a shoulder belt and alap belt is eliminated at the same time and thus the belt can betensioned.

Thus, when the locking means L moves in the B direction, the movementresistance is reduced by the movement auxiliary member 450, and thelocking means L can smoothly move in the B direction with respect to therail 415 as a whole. That is, when the knuckle 407 is retracted by thewire 421 and the slider 409 moves along the rail 415, the slider 409also somewhat undergoes a moment in a counterclockwise direction, asshown in FIG. 14. At this time, the contact body 453 comes in contactwith and rotates around the bottom wall 415 b of the rail 415, and thusthe movement resistance is reduced. With such a reduced movementresistance effect, the movement can be instantaneously made using asmall actuator.

At the same time, when the seat belt 11 receives the tension from theoccupant to allow a force to be exerted, the buckle 55 moves in the Adirection. In such a manner, the knuckle 407 is integrally retracted inthe same direction and rotates around the center pin 411. Therefore, thebite point P₁ of the other end 407 b of the knuckle 407 is bit into thebottom wall 415 b of the rail 415. With the bite force, the slider 409is raised relatively upward through the center pin 411. Therefore, thedeformable part 451 b of the support shaft 451 is easily deformed, andthe movement auxiliary member 450 having the support shaft 451 and thehollow cylindrical body 453 is broken. Therefore, a function to reducethe movement resistance of the movement auxiliary member 450 is degradedor lost. Accordingly, the buckle 55 is surely fixed so as not to move inthe A direction of the seat belt loosening direction. Therefore, theseat belt 11 can surely constrain the occupant.

If the emergency condition is avoided, even when the buckle 55 is lockeddue to the sudden deceleration, as described above, the buckle 55 isreversibly driven when the emergency condition is avoided, and thusreturns to an original state. That is, when the wire 421 is driven inthe A direction by the detection of the avoidance of the emergencycondition, the force in the A direction is exerted on the knuckle 407from the hook 425. Accordingly, the knuckle 407 rotates the center pin411 in a reverse direction a little, and bite locking at the bite pointP₁ is released. Next, the buckle 55 integrated with the knuckle 407moves in the A direction, and the seat belt 11 returns to the originallooseness state. Moreover, the broken movement auxiliary member 450 issuitably changed. However, when the emergency condition is a trivialone, naturally, the movement auxiliary member 450 will not reach thebroken state. In this case, a reversible operation is more smoothlyperformed.

As described above, the buckle 55 can reversibly be moved, and thus,even when the emergency condition is avoided, the occupant can continueto drive the vehicle with the seat belt 11 returned to the originallooseness state. Therefore, a coupling device with very high performanceis obtained. Further, with the movement auxiliary member 450, the buckle55 can be smoothly moved in the retraction direction B, and thus highperformance can be realized.

In addition, the motor 461 of the electromotive winch 460 for drivingthe wire 421 is, for example, a direct current (DC) motor. The number ofrotations of the direct current motor is proportional to the current tobe supplied to the direct current motor. When the direct current motorrotates forwardly, the current supplied to the direct current motor isproportional to the tension of the seat belt 11. In addition, the valueof the current flowing in the direct current motor is detected as thevalue of a voltage corresponding to the current by a current detectorprovided at the motor driving circuit and is outputted to the failuredetecting means 58, which is described below. The current of the directcurrent motor has relation to a rotational torque of the direct currentmotor, so that the rotational torque, that is, the tension given to theseat belt 11, can be estimated by using a load current value.

Moreover, the control configuration and sequence of the failuredetecting means 58 are the same manner as those described in the secondembodiment.

FOURTH EMBODIMENT

Next, a seat belt device 30 according to a fourth embodiment of theinvention will be described with reference to FIG. 17. FIG. 17 is adiagram schematically showing the configuration of an electromotive beltwinding device according to the fourth embodiment of the invention.

The seat belt device 70 is of a three-point support type and includes aseat belt 11 for constraining an occupant 1 to a seat 2, a retractor 71installed on a side surface of a vehicle body 3 for winding one end ofthe seat belt 11, a shoulder through 13 installed on an upper side ofthe side surface of the vehicle body 3 for turning down the seat belt 11in the vicinity of a shoulder of the occupant 1, a buckle stay part 72for detachably engaging a midportion of the seat belt 11 on a lower sideof the vehicle body 3 in the vicinity of a waist of the occupant, and alap anchor part 73 for fixing the other end of the seat belt 11 on alower side of the side surface of vehicle body 3.

The buckle stay part 72 includes a through tongue 74 for turning downthe seat belt 11, and a buckle 75 installed on the lower side of thevehicle body to be detachably engaged with the through tongue 74.

The seat belt device 70 further includes a tension detecting means 76and a tension varying means 77 provided at the buckle 75 of the bucklestay part 72, a failure detecting means 78 connected to the tensionvarying means 77 and the tension detecting means 76, and anattachment/detachment detecting means (buckle switch) 37 provided at thebuckle 55 for detecting the attachment/detachment of the through tongue74.

As described in the first embodiment, the tension detecting means 73includes a mounting structure of the anchor plate 211 and the base plate220, and a unit 230 in which the upper case 231 and the lower case 232are incorporated in the same manner. That is, the tension given by thetension varying means 77 described below is detected by the tensiondetecting means 76. The tension cannot be detected when the seat belt 11is not attached to the buckle stay part 72. For this reason, theattachment/detachment detecting means 37 of the seat belt 11 is providedat the buckle 75 in the same manner as in the second embodiment.

Next, the tension varying means 77 arranged between the buckle 75 andthe vehicle body will be described. In the drawing, the tension varyingmeans 77 includes an electromotive winch, serving as the tension varyingmeans 77 that retracts or protracts the seat belt 11 into or from thebuckle 75, and having a motor 81 and a reel 83, on which a wire 82connected to the tension detecting means 76 is wound.

Since the motor 81 rotates forwardly and reversely, the wire 82 can beretracted and protracted. A control unit 84 drives the motor 81 toeliminate loosening of the seat belt 11. In this case, by detecting thevalue of the current of the motor 84, the tension of the belt can alsobe estimated.

Though the retractor (belt winding device) 71 preferably has a forciblelocking mechanism and a pre-tensioner, as described in the firstembodiment, an electromotive retractor may be not provided.

In addition, the control configuration and sequence of the failuredetecting means 78 are the same as the second embodiment. Here, both thetension detecting means 76 and the tension varying means 77 are arrangedin series in the buckle stay part 72. Accordingly, a friction caused bythe guidance of the seat belt 11 has little effect, except for the errorof the tension detecting means 76 and the tension varying means 77. Forthis reason, failures of at least one of the tension detecting means 76and the tension varying means 77 can be accurately detected.

Moreover, though the preferred embodiments of the invention have beendescribed, the invention is not limited hereto. It should be understoodthat various changes and/or modifications can be made without departingfrom the spirit and scope of the invention.

1. A seat belt device comprising: a seat belt for constraining an occupant to a seat; a tension varying means allowing a tension given to the seat belt to vary; a tension detecting means for detecting a tension of the seat belt; and failure detecting means for comparing the tension given by the tension varying means to the tension detected by the tension detecting means, to detect failures of at least one of the tension varying means and the tension detecting means.
 2. The seat belt device according to claim 1, wherein the seat belt is of three-point support type, one end thereof being supported at an electromotive retractor in such a manner as to be freely wound, the other end thereof being supported at a lap anchor part, and the midportion thereof being supported at a buckle stay part, the tension varying means is provided at the electromotive retractor, and the tension detecting means is provided at the lap anchor part.
 3. The seat belt device according to claim 1, wherein the seat belt is of three-point support type, one end thereof being supported at an electromotive retractor in such a manner as to be freely wound, the other end thereof being supported at a lap anchor part, and the midportion thereof being supported at a buckle stay part, the tension varying means and the tension detecting means are arranged such that the tension varying means is provided at the retractor and the tension detecting means is provided at the buckle stay part, such that the tension varying means is provided at the buckle stay part and the tension detecting means is provided at the lap anchor part, or such that the tension varying means and the tension detecting means are provided at the buckle stay part, the buckle stay part is provided with an attachment/detachment detecting means for detecting an attachment of the seat belt, and the failure detecting means is allowed to be actuated when the attachment/detachment detecting means is detecting the attachment of the seat belt. 